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A Commentary on “Cortical Activity and the Explanatory Gap”
CONSCIOUSNESS AND COGNITION ARTICLE NO.
7, 169–172 (1998)
CC980342
A Commentary on ‘‘Cortical Activity and the Explanatory Gap’’ James W. Garson1 Philosophy Department, University of Houston, Houston, Texas 77006-3785
The explanatory gap is certainly one of the most puzzling and pressing problems about cognition. As is the case for all truly deep questions, the difficulty is not deciding what alternative to choose, but to see where to start. Our subjective experience provides a rich tapestry of tastes, smells, sights, and sounds, each with its own vibrant and compelling way of feeling. Our naive reaction is to balk at what Crick has called the Astonishing Hypothesis: that the unique richness of our sensory experience could be the mere product of the electrochemical activity of ten billion or so neurons between our ears. The fundamental problem in cognitive science, as I see it, is to explain how the Astonishing Hypothesis could be true. J. G. Taylor suggests we begin to close the gap by associating phenomenal experience with finitely enduring bubbles in a brand of recurrent neural network known to be implicated in sensory processing. The value of this work may not reside so much in the content of this speculative proposal, but instead in the methodological and conceptual presuppositions that it brings to light. Solving the puzzle of the explanatory gap demands speculation about the problem’s conceptual foundations. We need to take a closer look at Taylor’s unexamined assumptions. Putting them into question may help deepen our understanding of the shape of the problem. The problem is to explain how neural properties give rise to phenomenal properties, to the way things feel. Taylor’s strategy is to identify properties at the phenomenal level and project them ‘‘downward’’ to the corresponding features of neurological processing that explain them. But this projection is a very delicate matter. The guidance Taylor employs in carrying it out is based on folk intuitions which could easily lead us astray (as he is quick to admit). One of the most problematic of these is the thought that properties of subjective experience must arise from the same or similar properties in the neural substrate. That this strategy is dangerous in general can be easily illustrated. The hypothesis that the gene for straight hair is itself straight (or hairy) would not help explain why that gene results in straight hair. The wetness of water would not be explained by the wetness of individual water molecules. These cases are typical of many in science where properties at a higher level of description are emergent, that is, explaining them by attributing them to items at a lower level of explanation would be incorrect or incoherent. We must consider seriously the hypothesis that properties of phenomenal experience are emergent in the same way. If this is so, then Taylor’s whole projection strategy needs a major overhaul. Let me motivate worry about Taylor’s strategy with a series of explanatory fantaCommentary on J. G. Taylor (1998). Cortical activity and the explanatory gap. Consciousness and Cognition, 7(2), 109–148. 1 E-mail: [email protected]. 169 1053-8100/98 $25.00
Copyright 1998 by Academic Press All rights of reproduction in any form reserved.
170
JAMES W. GARSON
sies accompanied by instructive morals. It should be obvious that the first few commit the error of trying to explain higher level features of experience by inappropriately projecting these properties onto the neurology. The series of examples is intended to encourage the reader to apply to same morals to items further down the list— items which are discussed in Taylor’s paper. 1. Color perception requires that neurons in V5 themselves turn from one color to another, chameleon-like in registration with the colors that flit across their receptive fields. How else can the brain know what the colors really look like? Since it would otherwise be dark inside the skull, the brain needs a way to light those neurons to appreciate their colors. Or perhaps the visual neurons phosphoresce. Without brain illumination, there is an insurmountable optic gap. Moral. Representation of color need not be explained by the color of the representation. 2. When we feel a pain in our toe, we feel it in the toe not the brain. So the representation of pain in a toe must itself be in the toe. Since pain receptors are topographically mapped onto cortex, there must be another projective mechanism whereby the activity in that map is transported back to the toe ‘‘where it belongs.’’ Unless we find reverse pain pathways back to the toe, there will be an insurmountable neuralgic gap. Moral. Representation of location need not be explained by the location of the representation. 3. Our visual experience is bound together in a single image of the world. Yet we know that different aspects of visual processing (edge detection, motion, color, etc.) are carried out in separate topographic maps. Therefore, there must be some other map where information from these different centers is projected to form a unified visual map. Without the unified map, visual experience would be fragmented and there would be an insurmountable binding gap. Moral. Representation of binding need not be explained by the binding of representations. 4. We are unaware of the processes that account for our ability to hear, taste, smell, and see. Yet these neural activities are carried out in parts of the brain that are connected in one way or another to all its other parts. So there must be some mechanism or structure that filters out access of the conscious part of the brain to these details. Or perhaps the conscious processes are especially ‘‘loud’’ enduring or active so as to drown the others out. Without a method to block what we are unaware of, we would be aware of everything, and there would be an insurmountable ‘‘filtering’’ gap. Moral. Lack of representational access need not be explained by lack of access to representations. 5. Our sensory experience is smooth and continuous not grainy. We do not experience individual separate units of awareness. But, the neural system is particulate; that is, it is composed of many distinct neurons. So to explain the continuity of our experience, the neural structure must be continuous as well. Or perhaps it is actually particulate, but the brain gets fooled into the illusion of continuity because there are so many neurons. Without some feature of this kind there would be an insurmountable continuity gap.
COMMENTARY
171
Moral. The representation of continuity need not be explained by the continuity of representations. 6. Our sensory experience is immediate and direct. We experience a world out there, rather than as something presented to us indirectly by our perceptions of it. But, we know that all information about the world is presented to us by activity in the brain. Therefore, the perceptual information must present itself to the center of consciousness in such a strong and forceful way that the center ‘‘forgets’’ all about the fact that perception goes on in the brain. Without some forceful way to hide the fact that our experience of the world is indirect, there would be an insurmountable immediacy gap. Moral. Representation of immediacy need not be explained by the immediacy of representations. If Taylor’s projective strategy rests on conceptual confusions expressed in our morals, then why is it still so seductive? I submit that part of the problem lies in a residual homuncularism that is deeply rooted in our thoughts about the mind. When we learn that perception is a brain process, we feel compelled to locate consciousness in a ‘‘self’’ module (a homunculus or little man) which ‘‘views’’ the outputs of sensory activity. In order for this module to have a subjective experience of a colorful, unified, continuous, and immediate world, our intuitions now demand that the perceptual processes deliver a colorful, unified, continuous and immediately accessed ‘‘image’’ for the module to view. But, satisfying these intuitions does nothing to resolve the problem of the explanatory gap, since it is simply relocated inside the brain. The fundamental question now shifts to how neural activity brings about subjective experiences of the appropriate kind in the self module. Suppose, for example, that the perceptual system were known to deliver a perfectly continuous picture to the consciousness center, using a system of ‘‘sensory jelly’’ rather than neural arrays. Given that the self module still runs on particulate neurons, would the continuity of the picture it views in any way resolve the problem of how experience comes to be continuous? If the discontinuous nature of neural networks poses a difficulty in explaining the continuity of experience, then why is not the difficulty equally pressing that the receiver has a particulate rather than a continuous structure? The only answer I can formulate to this puzzle is to recognize that the continuity of experience is not itself to be explained by any form of continuity at the neural level. But, once this is admitted, there is no longer any motive to hunt for mechanisms that overcome the continuity problem as Taylor does. Let us come at this point from another direction by examining Taylor’s explanation for continuity of phenomenal experience in more detail. His answer (in Section 4.1) is that the feeling of continuity arises from a continuous sheet of large numbers of neurons. But, of course the sheet is still formed from particulate units, the neurons. That they happen to be laid down in what appears morphologically to be a continuous sheet cannot possibly explain the feeling of continuity. If the same neurons with the same synaptic connections were dissected out into a discontinuous collection would we expect feelings of continuity to disappear? Furthermore, the idea that continuity is generated by the sheer number of neurons seems far too crude. Do parts of our body (such as the back) that are poorly enervated feel relatively discontinuous? I grant that the number of neurons in a topographic map is related to the resolution
172
JAMES W. GARSON
of the sensory modality it serves. But good resolution and feelings of continuity are entirely different matters. The only motive for connecting the two I can fathom depends on a residual homuncularism. We imagine our self module viewing the topographic map with ‘‘eyes’’ of limited resolution. Then the numerous pixels (neurons) of the map become too small to resolve, yielding a continuous picture. But, it is far from clear that it makes any sense to postulate a self module that views the map, and even if there were one, the proposal that the map has a fine grain says nothing at all about whether the experience of that module is grainy or not. Why is a selfmodule attached to that array unable to experience the output from each neuron as a collection of discontinuous elements? And why can it not treat Taylor’s highestlevel qualia bubbles as a discontinuous collection? In raising these issues I do not intend to head off attempts at bridging the explanatory gap. I disagree with philosophers such as David Chalmers who believe that no conceivable advances in cognitive science can ever resolve the problem. But, I believe that Taylor’s methods are too crude. If we were to discover that the gene for straight hair is straight and the gene for curly hair is curly, we would be just as much in the dark as to how those genes governed the shape of hair. A full account would involve complex genetic mechanisms that never mention the shape of genes. It should be a working hypothesis in research on closing the explanatory gap that properties of experience are emergent, and need not and probably will not be wedded to the same or similar properties of neural arrays. Much more complex processes are certainly involved in the mechanisms that explain our phenomenal life. In any case, great care should be taken to fend off a residual homuncularism that would argue from the properties of topographic maps to corresponding properties of a self module that ‘‘views’’ those maps. It is this compelling but dangerous intuition that drives us to the conclusion that properties of the neural structure can explain corresponding properties of experience.
7, 169–172 (1998)
CC980342
A Commentary on ‘‘Cortical Activity and the Explanatory Gap’’ James W. Garson1 Philosophy Department, University of Houston, Houston, Texas 77006-3785
The explanatory gap is certainly one of the most puzzling and pressing problems about cognition. As is the case for all truly deep questions, the difficulty is not deciding what alternative to choose, but to see where to start. Our subjective experience provides a rich tapestry of tastes, smells, sights, and sounds, each with its own vibrant and compelling way of feeling. Our naive reaction is to balk at what Crick has called the Astonishing Hypothesis: that the unique richness of our sensory experience could be the mere product of the electrochemical activity of ten billion or so neurons between our ears. The fundamental problem in cognitive science, as I see it, is to explain how the Astonishing Hypothesis could be true. J. G. Taylor suggests we begin to close the gap by associating phenomenal experience with finitely enduring bubbles in a brand of recurrent neural network known to be implicated in sensory processing. The value of this work may not reside so much in the content of this speculative proposal, but instead in the methodological and conceptual presuppositions that it brings to light. Solving the puzzle of the explanatory gap demands speculation about the problem’s conceptual foundations. We need to take a closer look at Taylor’s unexamined assumptions. Putting them into question may help deepen our understanding of the shape of the problem. The problem is to explain how neural properties give rise to phenomenal properties, to the way things feel. Taylor’s strategy is to identify properties at the phenomenal level and project them ‘‘downward’’ to the corresponding features of neurological processing that explain them. But this projection is a very delicate matter. The guidance Taylor employs in carrying it out is based on folk intuitions which could easily lead us astray (as he is quick to admit). One of the most problematic of these is the thought that properties of subjective experience must arise from the same or similar properties in the neural substrate. That this strategy is dangerous in general can be easily illustrated. The hypothesis that the gene for straight hair is itself straight (or hairy) would not help explain why that gene results in straight hair. The wetness of water would not be explained by the wetness of individual water molecules. These cases are typical of many in science where properties at a higher level of description are emergent, that is, explaining them by attributing them to items at a lower level of explanation would be incorrect or incoherent. We must consider seriously the hypothesis that properties of phenomenal experience are emergent in the same way. If this is so, then Taylor’s whole projection strategy needs a major overhaul. Let me motivate worry about Taylor’s strategy with a series of explanatory fantaCommentary on J. G. Taylor (1998). Cortical activity and the explanatory gap. Consciousness and Cognition, 7(2), 109–148. 1 E-mail: [email protected]. 169 1053-8100/98 $25.00
Copyright 1998 by Academic Press All rights of reproduction in any form reserved.
170
JAMES W. GARSON
sies accompanied by instructive morals. It should be obvious that the first few commit the error of trying to explain higher level features of experience by inappropriately projecting these properties onto the neurology. The series of examples is intended to encourage the reader to apply to same morals to items further down the list— items which are discussed in Taylor’s paper. 1. Color perception requires that neurons in V5 themselves turn from one color to another, chameleon-like in registration with the colors that flit across their receptive fields. How else can the brain know what the colors really look like? Since it would otherwise be dark inside the skull, the brain needs a way to light those neurons to appreciate their colors. Or perhaps the visual neurons phosphoresce. Without brain illumination, there is an insurmountable optic gap. Moral. Representation of color need not be explained by the color of the representation. 2. When we feel a pain in our toe, we feel it in the toe not the brain. So the representation of pain in a toe must itself be in the toe. Since pain receptors are topographically mapped onto cortex, there must be another projective mechanism whereby the activity in that map is transported back to the toe ‘‘where it belongs.’’ Unless we find reverse pain pathways back to the toe, there will be an insurmountable neuralgic gap. Moral. Representation of location need not be explained by the location of the representation. 3. Our visual experience is bound together in a single image of the world. Yet we know that different aspects of visual processing (edge detection, motion, color, etc.) are carried out in separate topographic maps. Therefore, there must be some other map where information from these different centers is projected to form a unified visual map. Without the unified map, visual experience would be fragmented and there would be an insurmountable binding gap. Moral. Representation of binding need not be explained by the binding of representations. 4. We are unaware of the processes that account for our ability to hear, taste, smell, and see. Yet these neural activities are carried out in parts of the brain that are connected in one way or another to all its other parts. So there must be some mechanism or structure that filters out access of the conscious part of the brain to these details. Or perhaps the conscious processes are especially ‘‘loud’’ enduring or active so as to drown the others out. Without a method to block what we are unaware of, we would be aware of everything, and there would be an insurmountable ‘‘filtering’’ gap. Moral. Lack of representational access need not be explained by lack of access to representations. 5. Our sensory experience is smooth and continuous not grainy. We do not experience individual separate units of awareness. But, the neural system is particulate; that is, it is composed of many distinct neurons. So to explain the continuity of our experience, the neural structure must be continuous as well. Or perhaps it is actually particulate, but the brain gets fooled into the illusion of continuity because there are so many neurons. Without some feature of this kind there would be an insurmountable continuity gap.
COMMENTARY
171
Moral. The representation of continuity need not be explained by the continuity of representations. 6. Our sensory experience is immediate and direct. We experience a world out there, rather than as something presented to us indirectly by our perceptions of it. But, we know that all information about the world is presented to us by activity in the brain. Therefore, the perceptual information must present itself to the center of consciousness in such a strong and forceful way that the center ‘‘forgets’’ all about the fact that perception goes on in the brain. Without some forceful way to hide the fact that our experience of the world is indirect, there would be an insurmountable immediacy gap. Moral. Representation of immediacy need not be explained by the immediacy of representations. If Taylor’s projective strategy rests on conceptual confusions expressed in our morals, then why is it still so seductive? I submit that part of the problem lies in a residual homuncularism that is deeply rooted in our thoughts about the mind. When we learn that perception is a brain process, we feel compelled to locate consciousness in a ‘‘self’’ module (a homunculus or little man) which ‘‘views’’ the outputs of sensory activity. In order for this module to have a subjective experience of a colorful, unified, continuous, and immediate world, our intuitions now demand that the perceptual processes deliver a colorful, unified, continuous and immediately accessed ‘‘image’’ for the module to view. But, satisfying these intuitions does nothing to resolve the problem of the explanatory gap, since it is simply relocated inside the brain. The fundamental question now shifts to how neural activity brings about subjective experiences of the appropriate kind in the self module. Suppose, for example, that the perceptual system were known to deliver a perfectly continuous picture to the consciousness center, using a system of ‘‘sensory jelly’’ rather than neural arrays. Given that the self module still runs on particulate neurons, would the continuity of the picture it views in any way resolve the problem of how experience comes to be continuous? If the discontinuous nature of neural networks poses a difficulty in explaining the continuity of experience, then why is not the difficulty equally pressing that the receiver has a particulate rather than a continuous structure? The only answer I can formulate to this puzzle is to recognize that the continuity of experience is not itself to be explained by any form of continuity at the neural level. But, once this is admitted, there is no longer any motive to hunt for mechanisms that overcome the continuity problem as Taylor does. Let us come at this point from another direction by examining Taylor’s explanation for continuity of phenomenal experience in more detail. His answer (in Section 4.1) is that the feeling of continuity arises from a continuous sheet of large numbers of neurons. But, of course the sheet is still formed from particulate units, the neurons. That they happen to be laid down in what appears morphologically to be a continuous sheet cannot possibly explain the feeling of continuity. If the same neurons with the same synaptic connections were dissected out into a discontinuous collection would we expect feelings of continuity to disappear? Furthermore, the idea that continuity is generated by the sheer number of neurons seems far too crude. Do parts of our body (such as the back) that are poorly enervated feel relatively discontinuous? I grant that the number of neurons in a topographic map is related to the resolution
172
JAMES W. GARSON
of the sensory modality it serves. But good resolution and feelings of continuity are entirely different matters. The only motive for connecting the two I can fathom depends on a residual homuncularism. We imagine our self module viewing the topographic map with ‘‘eyes’’ of limited resolution. Then the numerous pixels (neurons) of the map become too small to resolve, yielding a continuous picture. But, it is far from clear that it makes any sense to postulate a self module that views the map, and even if there were one, the proposal that the map has a fine grain says nothing at all about whether the experience of that module is grainy or not. Why is a selfmodule attached to that array unable to experience the output from each neuron as a collection of discontinuous elements? And why can it not treat Taylor’s highestlevel qualia bubbles as a discontinuous collection? In raising these issues I do not intend to head off attempts at bridging the explanatory gap. I disagree with philosophers such as David Chalmers who believe that no conceivable advances in cognitive science can ever resolve the problem. But, I believe that Taylor’s methods are too crude. If we were to discover that the gene for straight hair is straight and the gene for curly hair is curly, we would be just as much in the dark as to how those genes governed the shape of hair. A full account would involve complex genetic mechanisms that never mention the shape of genes. It should be a working hypothesis in research on closing the explanatory gap that properties of experience are emergent, and need not and probably will not be wedded to the same or similar properties of neural arrays. Much more complex processes are certainly involved in the mechanisms that explain our phenomenal life. In any case, great care should be taken to fend off a residual homuncularism that would argue from the properties of topographic maps to corresponding properties of a self module that ‘‘views’’ those maps. It is this compelling but dangerous intuition that drives us to the conclusion that properties of the neural structure can explain corresponding properties of experience.